A few months ago, I wrote an explanation about the “AQL“: what it is and how to use it. I also listed a few frequent questions, but it seems like I forgot one of them:
What AQL tolerance is suitable to my products?
Unfortunately, it is hard to base this on statistical reasoning. So I don’t have any definitive answer.
The right AQL depends on three things:
- The market you are selling into,
- The kind of risk the users run by using/consuming/getting close to your product,
- The amount of efforts spent at the new product development phase.
1. What your market/customers can accept
The most common AQL chosen by importers is 2.5% for major defects, 4.0 for minor defects, and 0.1 for critical defects. It is considered the “standard” tolerance for most consumer products sold in supermarkets in North America and in Europe.
Based on this standard, you can adjust an AQL that is a bit stricter (say, 1.0/2.5/0.10) if you sell your products in a high-end boutique channel. Or a bit looser (say, 4.0/6.5/0.1) for sale on a particularly low-end market.
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2. The user risk
For many car and plane parts, and for pharmaceuticals, the accepted defect rate is much lower than 1%.
That’s even more true when a bad part might cause injury or death. There is no way to set an AQL in that case — it always has to be a zero tolerance.
They also want to reduce the proportion of products with non-safety-related failures. AQL limits of 1.0 or higher are not realistic here. Those failures have to be very low, since they’ve are so costly (e.g. having to call a car back for repair).
To do this, they have to spend efforts on:
- Risk analyses and accelerated reliability testing on the design (at the drawing and prototyping stages);
- Good process engineering and good testing stations;
- Great process controls during manufacturing;
- Intuitive interfaces for users, warnings when necessary, etc.
3. Efforts spent at the new product development phase
Let’s take the example of the consumer electronics industry. When a company spends, say, a million USD developing a new product that aims at sales of 500,000 units a year, they negotiate lower limits.
It depends on the amount of efforts they have spent (see section 2 above) and on product complexity. They can usually aim for these limits:
- Low to average efforts: 1.0% for major defects
- Average to high efforts: 0.6% for major defects
- Very high efforts: 0.25% for major defects
To sum up:
There are no guidelines for deciding what AQL limits to choose. You have to decide what your tolerance will be. If the whole batch should not contain more than 1.0% of a certain kind of defect (over the long run), then the AQL should be 1.0% for this kind of defect.
Also, keep in mind, the AQL limit is all about the manufacturing quality, just after production is over. You also need to factor in your product’s reliability & durability.
Is it clear? Let me know if you have any questions by leaving a comment, please.
Additional resource >> Learn even more about QC, read our detailed Quality Control basic concepts post here.
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